1. Field of the Invention
This invention relates generally to ceramics and more particularly to a composite article comprising a luminescent ceramic material and an interspersed reflective material formed by cosintering a first ceramic material and a second material.
2. Description of the Related Art
A luminescent material absorbs energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Most luminescent materials, also known as xe2x80x9cphosphorsxe2x80x9d or xe2x80x9cscintillatorsxe2x80x9d, emit radiation in the visible portion of the spectrum in response to the absorption of radiation outside the visible portion of the spectrum. Most phosphors are responsive to portions of the electromagnetic spectrum which are more energetic than the visible portion of the spectrum. For example, there are phosphors which are responsive to ultraviolet light (as in fluorescent lamps), electrons (as in cathode ray tubes) and x-rays (as in radiography).
One example of an application using luminescent materials is a computed tomography (CT) scanner. In a CT scanner, an x-ray source and an x-ray detector array are positioned on opposite sides of a subject and rotated around the subject in fixed relation to each other. CT scanners typically include a solid scintillator which comprises a luminescent material in the form of a transparent solid body. A typical detector array in a CT scanner includes a plurality of individual scintillator bars positioned side-by-side with an individual photodetector diode coupled to each scintillator bar to convert its luminescent light into a corresponding electrical signal. The scintillator material of a cell absorbs x-rays incident on that cell and emits light which is collected by a photodetector for that cell. During data collection, each cell of the detector array provides an output signal representative of the present light intensity in that cell of the array. The luminescent material in a CT scanner typically has a linear characteristic in which the light output is a linear function of the amount of absorbed radiation such that the light output can be directly correlated with the intensity of stimulating radiation. The output signals are processed to create an image of the subject in a manner which is well known in the CT scanner art.
It is generally advantageous to provide a reflective coating on the surfaces of the scintillator bar other than the surface on which the photodetector diode is located. The coating reflects the light produced in the individual scintillator bars of the array. The reflective properties of the coating improve the accuracy of the resulting image by preventing light from propagating from one scintillator bar to another (commonly referred to as xe2x80x9ccross talkxe2x80x9d).
Various methods are known in the art for forming a detector array comprising a number of scintillator bars separated by reflective layers. Typically, the scintillator material is produced by preparing an appropriate ceramic powder, milling the powder, and pressing the powder to form a wafer. The wafer is then sintered, typically to a density greater than 99% of theoretical density.
One problem which is known to occur with conventional scintillator forming methods is that during the sintering step, the wafer may warp. The warping is due to differential shrinkage caused by an inhomogeneous density distribution in the scintillator material. Because the dimensional tolerances for scintillator bars in some applications are very small, e.g. less than 0.0013 cm (0.0005 inch), differential shrinkage during sintering of the scintillator bars can make the final dimensions of the bars very difficult to achieve without further processing. Thus, the scintillator wafer is typically ground, lapped, and polished to the desired dimensions after sintering, which can be costly and time consuming. The process of providing a reflective coating on surfaces of the scintillator bars may also be costly, as it can involve many process steps.
Other methods of forming detector arrays are known. For example, German Patent No. 19709690 A1 discloses a ceramic element with a layer structure containing alternating layers of high density and high porosity. The porous layers include microstructures which form bridges between the adjacent layers of high density. This method, however, results in a detector array in which the scintillator elements are optically coupled by the bridges, which produces cross talk, decreasing the imaging accuracy of the device.
German Patent No. 19709691 A1 discloses a process for the manufacture of a structured ceramic element in which green ceramic elements provided with spacer structures are stacked and sintered. A function assisting material can be added to the cavities between the spacer structures after the green ceramic elements are sintered. However, again, the spacer elements optically couple the ceramic elements, which decreases the imaging accuracy of the device.
It would be desirable, therefore, to have a method of easily and effectively forming a detector array including a reflective layer interspersed between scintillator bars, while avoiding problems found in known methods.
A method of forming a composite article, according to an exemplary embodiment of the invention, comprises the steps of forming a plurality of green ceramic elements, wherein the green ceramic elements are arranged side by side, and the green ceramic elements are spaced from each other by gaps; filling the gaps with a second material; and sintering the green ceramic elements with the second material to form the composite article. The second material, after being sintered, acts as a reflector layer to prevent substantially all light in one of the sintered ceramic elements from reaching an adjacent sintered ceramic element. The plurality of green ceramic elements may be formed by injection molding and may each extend from a common connecting member.
The step of filling the gaps may be carried out by forming a slurry containing the second material in powder form and immersing the green ceramic elements in the slurry. The step of filling the gaps may also be carried out by spraying the second material in powder form into the gaps. The second material which fills the gaps can be the same material as that used to form the green ceramic elements, but having a different pack density and/or particle size, for example.
The process of cosintering the green ceramic elements with the reflector composition provides improved dimensional control during sintering and reduces processing costs.